The present invention relates to pressure gauges, and relates more particularly to a pressure gauge reset mechanism which accurately zeroes the reading of the pressure gauge after each measurement.
FIG. 1 shows a pressure gauge reset mechanism according to the prior art which is generally comprised of a cylinder (1) having a pressure measuring head (10) at one end, and a pressure gauge (2) coupled to the cylinder (1). The pressure gauge (2) comprises an index shaft with a gear (20), and a torsional spring (21) mounted on the inside to hold down the index shaft with a gear (20). The torsional spring (21) has one end stopped at the inside of the pressure gauge (2), and an opposite end extending out of the pressure gauge (2) and fixedly connected with a button (22). The cylinder (1) comprises a piston (11) having a head (110) and a rod (111) perpendicularly extending from the head (110) at one side, a first spring member (12) mounted on the inside to force the piston (11) upwards toward the pressure measuring head (10), a pressure rod (14) having a toothed portion 140 extending out of the cylinder (1) and meshed with the gear of the index shaft (20), and a second spring member (13) having a bottom end stopped at the top end of the pressure rod (14) and a top end coupled to the rod (111) of the piston (11) (see also FIG. 2). When the pressure measuring head 10 of the cylinder 1 is attached to the air valve of the tire to be measured, the piston (11) is forced downwards by the pressure of the tire, causing the second spring member (13) to force the pressure rod (14) out of the cylinder (1), therefore the index shaft (20) is turned by the pressure rod (14) to move the index. When the button (22) is depressed after each measurement, the torsional spring (21) releases the index shaft (20) and therefore the pressure rod is forced by the second spring (13) to return to its initial position. When the pressure rod (14) is pulled upwards to its former position, the index shaft (20) is turned to zero the reading of the index. This structure of pressure gauge reset mechanism is functional, however it is not durable in use because the second spring member (13) tends to wear with use. If the second spring member (13) wears, the pressure rod (14) cannot be accurately returned to its former position, causing the index shaft (20) unable to accurately zero the reading of the index.